As disclosed by U.S. Pat. Nos. 3,678,355 and 3,826,928, it is desirable to provide an A.C. motor control using solid state technique to acquire a tight, sophisticated control of motor acceleration, deceleration and programmed speed which, to all intents and purposes, eliminates lag in motor response to change speed signals in the control system. The desired result is instantaneous response in motor power or electrical braking, such as could be advantageously utilized in elevator motors.
The control circuit therefore provides motor directional and speed reference signals as voltages shaped into a smooth linear ramp summed with a feedback signal which is responsive to motor speed and direction. It is desirable for the amplitude and polarity of the amplified error to control the current flow and reversing of phase rotation, respectively, in at least two of the three phase motor inputs through fast responding, solid state trigger circuits controlling thyristor switches. Motor direction and speed conditions are sensed by a special A.C. tachometer and feedback circuit which feeds into the feed control circuit, as described above. The A.C. tachometer with a direction and speed sensing circuit is the subject matter of and is fully disclosed in a copending application Ser. No. 875,193, filed Nov. 10, 1969, now U.S. Pat. No. 3,614,616.